Spring IOC ( 一 ) : DefaultSingletonBeanRegistry源码分析

这是DefaultSingletonBeanRegistry类的体系结构,由一个类一个责任的原则

• AliasRegistry : 提供别名注册的接口
• SingletonBeanRegistry :  提供单例bean注册的接口
• ObjectFactory : 这个接口通常用于封装一个通用的工厂，它只有一个方法getObject() ,它调用getObject()方法返回一个新的实例，一些在每次调用的目标对象（原型）.
• DisposableBean : 接口实现为beans要破坏释放资源。它也只有一个方法destroy(),由一个破坏一个singleton的BeanFactory调用。
• SimpleAliasRegistry: 它简单地实现了AliasRegistry接口。
• DefaultSingletonBeanRegistry:它继承SimpleAliasRegistry类和实现了SingletonBeanRegistry接口，因此这个类可以有别名注册的功能和单例bean注册的功能，并且他还支持注册DisposableBean实例;它依赖ObjectFactory接口和DisposableBean接口(关闭注册表时调用到了destroy方法)。

 /**
  * 共享bean实例的通用注册表，实现了SingletonBeanRegistry. 允许注册表中注册的单例应该被所有调用者共享，通过bean名称获得。
  *
  * 还支持登记的DisposableBean实例，（这可能会或不能正确的注册单例），关闭注册表时destroyed.
  * 可以注册bean之间的依赖关系，执行适当的关闭顺序。
  *
  *
  * 这个类主要用作基类的BeanFactory实现， 提供基本的管理
  * singleton bean 实例功能。
  *
  */
 public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements
         SingletonBeanRegistry {

 //内部标记为一个空的单例对象： 并发 Maps( 不支持空值 )作为标志值。
     protected static final Object NULL_OBJECT = new Object();

 // 日记用来记录子类
     protected final Log logger = LogFactory.getLog(getClass());

 //是存放singleton对象的缓存
     private final Map singletonObjects = new ConcurrentHashMap();

 // 是存放制造singleton的工厂对象的缓存
     private final Map singletonFactories = new HashMap();

 //是存放singletonFactory 制造出来的 singleton 的缓存
     private final Map earlySingletonObjects = new HashMap();
 //以上三个缓存是这个类存放单例bean的主要Map
 //就是单例注册表
     private final Set registeredSingletons = new LinkedHashSet(
             16);

 //目前正在创建中的单例bean的名称的集合
     private final Set singletonsCurrentlyInCreation = Collections
             .synchronizedSet(new HashSet());

 //存放异常出现的相关的原因的集合
     private Set suppressedExceptions;

 //标志，指示我们目前是否在销毁单例中
     private boolean singletonsCurrentlyInDestruction = false;

 //存放一次性bean的缓存
     private final Map disposableBeans = new LinkedHashMap();

 //外部bean与被包含在外部bean的所有内部bean集合包含关系的缓存
     private final Map> containedBeanMap = new ConcurrentHashMap>();

 //指定bean与依赖指定bean的所有bean的依赖关系的缓存
     private final Map> dependentBeanMap = new ConcurrentHashMap>();

 //指定bean与创建这个bean所需要依赖的所有bean的依赖关系的缓存
     private final Map> dependenciesForBeanMap = new ConcurrentHashMap>();

     // SingletonBeanRegistry接口的registerSingleton方法的实现
     public void registerSingleton(String beanName, Object singletonObject)
             throws IllegalStateException {
         Assert.notNull(beanName, "'beanName' must not be null");
         synchronized (this.singletonObjects) {
             Object oldObject = this.singletonObjects.get(beanName);
 //如果singletonObjects缓存找到有指定名称为beanName的对象，则表示该名称已被占用
             if (oldObject != null) {
                 throw new IllegalStateException("Could not register object ["
                         + singletonObject + "] under bean name '" + beanName
                         + "': there is already object [" + oldObject
                         + "] bound");
             }
 //若该名称没被占用，真正的注册操作在这里实现
             addSingleton(beanName, singletonObject);
         }
     }


     protected void addSingleton(String beanName, Object singletonObject) {
         synchronized (this.singletonObjects) {
             // 因为singletonObjects类型是ConcurrentHashMap,并发Map不支持空值作为标志值，所以用NULL_OBJECT来代替
             this.singletonObjects.put(beanName,
                     (singletonObject != null ? singletonObject : NULL_OBJECT));
             // beanName已被注册存放在singletonObjects缓存，那么singletonFactories不应该再持有名称为beanName的工厂
             this.singletonFactories.remove(beanName);
             // beanName已被注册存放在singletonObjects缓存，那么earlySingletonObjects不应该再持有名称为beanName的bean。
             this.earlySingletonObjects.remove(beanName);
             // beanName放进单例注册表中
             this.registeredSingletons.add(beanName);
         }
     }

     /**
      * 添加 名称为beanName的singletonFactory对象
      *
      */
     protected void addSingletonFactory(String beanName,
             ObjectFactory singletonFactory) {
         Assert.notNull(singletonFactory, "Singleton factory must not be null");
         synchronized (this.singletonObjects) {
             // 判断singletonObjects内名字为beanName是否被占用，若没有，进行注册操作
             if (!this.singletonObjects.containsKey(beanName)) {
                 this.singletonFactories.put(beanName, singletonFactory);
                 this.earlySingletonObjects.remove(beanName);
                 this.registeredSingletons.add(beanName);
             }
         }
     }

     // SingletonBeanRegistry接口的getSingleton方法的实现
     public Object getSingleton(String beanName) {
         return getSingleton(beanName, true);
     }


     protected Object getSingleton(String beanName, boolean allowEarlyReference) {
         Object singletonObject = this.singletonObjects.get(beanName);
         // 如果singletonObjects指定beanName的对象是不存在的
         if (singletonObject == null) {
             synchronized (this.singletonObjects) {
                 singletonObject = this.earlySingletonObjects.get(beanName);
                 // 如果earlySingletonObjects指定的beanName的对象是不存在的且allowEarlyReference是允许的
                 if (singletonObject == null && allowEarlyReference) {
                     ObjectFactory singletonFactory = this.singletonFactories
                             .get(beanName);
                     // 如果存在指定beanName的singletonFactory对象
                     if (singletonFactory != null) {
                         // singletonFactory创建指定的单例对象
                         singletonObject = singletonFactory.getObject();
                         // 这里可以看出earlySingletonObjects缓存应该是存放singletonFactory产生的singleton
                         this.earlySingletonObjects.put(beanName,
                                 singletonObject);
                         // 这里表示指定的beanName已被占用，所以要在singletonFactories移除该名称
                         this.singletonFactories.remove(beanName);
                     }
                 }
             }
         }
         return (singletonObject != NULL_OBJECT ? singletonObject : null);
     }


     public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
         Assert.notNull(beanName, "'beanName' must not be null");
         synchronized (this.singletonObjects) {
             Object singletonObject = this.singletonObjects.get(beanName);
             // 如果singetonObjects缓存不存在名称为beanName的对象
             if (singletonObject == null) {
                 // 如果目前在销毁singellton
                 if (this.singletonsCurrentlyInDestruction) {
                     throw new BeanCreationNotAllowedException(
                             beanName,
                             "Singleton bean creation not allowed while the singletons of this factory are in destruction "
                                     + "(Do not request a bean from a BeanFactory in a destroy method implementation!)");
                 }
                 if (logger.isDebugEnabled()) {
                     logger.debug("Creating shared instance of singleton bean '"
                             + beanName + "'");
                 }
                 // 单例对象创建前的回调,默认实现注册正在创建的单例
                 beforeSingletonCreation(beanName);
                 // 判断存储异常相关原因的集合是否已存在
                 boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
                 // 若没有，刚创建异常集合的实例
                 if (recordSuppressedExceptions) {
                     this.suppressedExceptions = new LinkedHashSet();
                 }
                 try {
                     // 由参数给定的singletonFactory创建singleton对象,getObject方法的具体实现由ObjectFactory的子类决定
                     singletonObject = singletonFactory.getObject();
                 } catch (BeanCreationException ex) {
                     // 如果异常被抓取，在这里将出现异常的原因抛出
                     if (recordSuppressedExceptions) {
                         for (Exception suppressedException : this.suppressedExceptions) {
                             ex.addRelatedCause(suppressedException);
                         }
                     }
                     throw ex;
                 } finally {
                     // 结束前，将异常集合销毁掉
                     if (recordSuppressedExceptions) {
                         this.suppressedExceptions = null;
                     }
                     // 单例创建之后的回调,默认的实现标志单例不要在创建了。
                     afterSingletonCreation(beanName);
                 }
                 // 注册创建后的单例
                 addSingleton(beanName, singletonObject);
             }
             return (singletonObject != NULL_OBJECT ? singletonObject : null);
         }
     }

     /**
      * 注册 发生在singeton bean 实例创建之间发生的异常
      */
     protected void onSuppressedException(Exception ex) {
         synchronized (this.singletonObjects) {
             if (this.suppressedExceptions != null) {
                 this.suppressedExceptions.add(ex);
             }
         }
     }

     /**
      * 移除名称为beanName的单例,主要在四个集合中移除，
      * 如singletonObjects,singletonFactories,earlySingletonObjects
      * ,registeredSingletons
      *
      */
     protected void removeSingleton(String beanName) {
         synchronized (this.singletonObjects) {
             this.singletonObjects.remove(beanName);
             this.singletonFactories.remove(beanName);
             this.earlySingletonObjects.remove(beanName);
             this.registeredSingletons.remove(beanName);
         }
     }

     // singletonBeanRegistry接口的containsSingleton方法实现
     public boolean containsSingleton(String beanName) {
         return (this.singletonObjects.containsKey(beanName));
     }

     // singletonBeanRegistry接口的getSingletonNames方法实现
     public String[] getSingletonNames() {
         // 对singletonObjects加锁，可能是为了防止registeredSingletons和singletonObjects出现不一致的问题
         synchronized (this.singletonObjects) {
             return StringUtils.toStringArray(this.registeredSingletons);
         }
     }

     // singletonBeanRegistry接口的getSingletonCount方法实现
     public int getSingletonCount() {
         synchronized (this.singletonObjects) {
             return this.registeredSingletons.size();
         }
     }

     /**
      * 单例对象创建前的回调,默认实现singletonsCurrentlyInCreation集合注册正在创建的单例.
      *
      */
     protected void beforeSingletonCreation(String beanName) {
         if (!this.singletonsCurrentlyInCreation.add(beanName)) {
             throw new BeanCurrentlyInCreationException(beanName);
         }
     }

     /**
      * 单例创建之后的回调,默认实现singletonCurrentlyInCreation集合移除正在创建的单例
      *
      */
     protected void afterSingletonCreation(String beanName) {
         if (!this.singletonsCurrentlyInCreation.remove(beanName)) {
             throw new IllegalStateException("Singleton '" + beanName
                     + "' isn't currently in creation");
         }
     }

     /**
      * 返回 存放正在创建单例的集合是否包含指定名称为beanName的单例存在
      *
      */
     public final boolean isSingletonCurrentlyInCreation(String beanName) {
         return this.singletonsCurrentlyInCreation.contains(beanName);
     }

     /**
      * 一次性bean注册，存放在disponsableBeans集合中
      *
      */
     public void registerDisposableBean(String beanName, DisposableBean bean) {
         synchronized (this.disposableBeans) {
             this.disposableBeans.put(beanName, bean);
         }
     }

     /**
      * 注册两个bean之间的控制关系,例如内部bean和包含其的外部bean之间
      *
      */
     public void registerContainedBean(String containedBeanName,
             String containingBeanName) {
         synchronized (this.containedBeanMap) {
             // 从containedBeanMap缓存中查找外部bean名为containingBeanName的内部bean集合
             Set containedBeans = this.containedBeanMap
                     .get(containingBeanName);
             // 如果没有，刚新建一个存放内部bean的集合，并且存放在containedBeanMap缓存中
             if (containedBeans == null) {
                 containedBeans = new LinkedHashSet(8);
                 this.containedBeanMap.put(containingBeanName, containedBeans);
             }
             // 将名为containedBeanName的内部bean存放到内部bean集合
             containedBeans.add(containedBeanName);
         }
         // 紧接着调用注册内部bean和外部bean的依赖关系的方法
         registerDependentBean(containedBeanName, containingBeanName);
     }

     /**
      * 注册给定bean的一个依赖bean，给定的bean销毁之前被销毁。
      *
      */
     public void registerDependentBean(String beanName, String dependentBeanName) {
         // 调用SimpleAliasRegistry的canonicalName方法，这方法是将参数beanName当做别名寻找到注册名，并依此递归
         String canonicalName = canonicalName(beanName);
         synchronized (this.dependentBeanMap) {
             // 从dependentBeanMap缓存中找到依赖名为canonicalName这个bean的 依赖bean集合
             Set dependentBeans = this.dependentBeanMap
                     .get(canonicalName);
             // 如果为空，则新建一个依赖bean集合，并且存放到dependentBeanMap缓存中
             if (dependentBeans == null) {
                 dependentBeans = new LinkedHashSet(8);
                 this.dependentBeanMap.put(canonicalName, dependentBeans);
             }
             // 依赖bean集合添加参数2指定的dependentBeanName
             dependentBeans.add(dependentBeanName);
         }
         synchronized (this.dependenciesForBeanMap) {
             // 从dependenciesForBeanMap缓存中找到dependentBeanName要依赖的所有bean集合
             Set dependenciesForBean = this.dependenciesForBeanMap
                     .get(dependentBeanName);
             if (dependenciesForBean == null) {
                 dependenciesForBean = new LinkedHashSet(8);
                 this.dependenciesForBeanMap.put(dependentBeanName,
                         dependenciesForBean);
             }
             dependenciesForBean.add(canonicalName);
         }
     }

     /**
      *  确定是否还存在名为beanName的被依赖关系
      */
     protected boolean hasDependentBean(String beanName) {
         return this.dependentBeanMap.containsKey(beanName);
     }

     /**
      * 返回依赖于指定的bean的所有bean的名称，如果有的话。
      *
      */
     public String[] getDependentBeans(String beanName) {
         Set dependentBeans = this.dependentBeanMap.get(beanName);
         if (dependentBeans == null) {
             return new String[0];
         }
         return StringUtils.toStringArray(dependentBeans);
     }

     /**
      * 返回指定的bean依赖于所有的bean的名称，如果有的话。
      *
      */
     public String[] getDependenciesForBean(String beanName) {
         Set dependenciesForBean = this.dependenciesForBeanMap
                 .get(beanName);
         // 如果没有的话返回new String[0]而不是null
         if (dependenciesForBean == null) {
             return new String[0];
         }
         return dependenciesForBean.toArray(new String[dependenciesForBean
                 .size()]);
     }

     // 销毁单例
     public void destroySingletons() {
         if (logger.isInfoEnabled()) {
             logger.info("Destroying singletons in " + this);
         }
         // 单例目前销毁标志开始
         synchronized (this.singletonObjects) {
             this.singletonsCurrentlyInDestruction = true;
         }

         // 销毁disponsableBeans缓存中所有单例bean
         synchronized (this.disposableBeans) {
             String[] disposableBeanNames = StringUtils
                     .toStringArray(this.disposableBeans.keySet());
             for (int i = disposableBeanNames.length - 1; i >= 0; i--) {
                 destroySingleton(disposableBeanNames[i]);
             }
         }

         // containedBeanMap缓存清空,dependentBeanMap缓存清空，dependenciesForBeanMap缓存清空
         this.containedBeanMap.clear();
         this.dependentBeanMap.clear();
         this.dependenciesForBeanMap.clear();

         // singeltonObjects缓存清空，singletonFactories缓存清空，earlySingletonObjects缓存清空，registeredSingletons缓存清空
         synchronized (this.singletonObjects) {
             this.singletonObjects.clear();
             this.singletonFactories.clear();
             this.earlySingletonObjects.clear();
             this.registeredSingletons.clear();
             // 单例目前正在销毁标志为结束
             this.singletonsCurrentlyInDestruction = false;
         }
     }


     public void destroySingleton(String beanName) {
         // Remove a registered singleton of the given name, if any.
         removeSingleton(beanName);

         // Destroy the corresponding DisposableBean instance.
         DisposableBean disposableBean;
         synchronized (this.disposableBeans) {
             disposableBean = (DisposableBean) this.disposableBeans
                     .remove(beanName);
         }
         destroyBean(beanName, disposableBean);
     }


     protected void destroyBean(String beanName, DisposableBean bean) {
         // Trigger destruction of dependent beans first...
         // 这段代码告诉我们先移除要销毁依赖bean
         Set dependencies = this.dependentBeanMap.remove(beanName);
         if (dependencies != null) {
             if (logger.isDebugEnabled()) {
                 logger.debug("Retrieved dependent beans for bean '" + beanName
                         + "': " + dependencies);
             }
             for (String dependentBeanName : dependencies) {
                 destroySingleton(dependentBeanName);
             }
         }

         // Actually destroy the bean now...
         // 销毁bean实例
         if (bean != null) {
             try {
                 bean.destroy();
             } catch (Throwable ex) {
                 logger.error("Destroy method on bean with name '" + beanName
                         + "' threw an exception", ex);
             }
         }

         // Trigger destruction of contained beans...
         // 从containedBeanMap缓存中移除要销毁的bean，递归移除它的包含内部bean集合
         Set containedBeans = this.containedBeanMap.remove(beanName);
         if (containedBeans != null) {
             for (String containedBeanName : containedBeans) {
                 destroySingleton(containedBeanName);
             }
         }

         // Remove destroyed bean from other beans' dependencies.
         // 从其它bean的依赖bean集合中移除要销毁的bean
         synchronized (this.dependentBeanMap) {
             for (Iterator>> it = this.dependentBeanMap
                     .entrySet().iterator(); it.hasNext();) {
                 Map.Entry> entry = it.next();
                 Set dependenciesToClean = entry.getValue();
                 dependenciesToClean.remove(beanName);
                 if (dependenciesToClean.isEmpty()) {
                     it.remove();
                 }
             }
         }

         // Remove destroyed bean's prepared dependency information.
         // 最后 从dependenciesForBeanMap缓存中移除要销毁的bean
         this.dependenciesForBeanMap.remove(beanName);
     }

     /**
      * Expose the singleton mutex to subclasses.
      *
      * Subclasses should synchronize on the given Object if they perform any
      * sort of extended singleton creation phase. In particular, subclasses
      * should not have their own mutexes involved in singleton creation,
      * to avoid the potential for deadlocks in lazy-init situations.
      */
     protected final Object getSingletonMutex() {
         return this.singletonObjects;
     }

 }